1. Field of the Invention
The present invention relates to a keypad used for electronic equipment such as mobile telephones, and more particularly to an illuminated button with its key button illuminated from its bottom by a light source.
2. Description of Related Art
Mobile telephones are rapidly spreading thanks to the remarkable progress in technology, and expected to further spread because of their convenience capable of anytime, anywhere communications. To achieve the anytime, anywhere communications, they employ an illuminated button pad to enable users to operate even in the night or in dark places.
FIG. 12 is a perspective view showing an appearance of a mobile telephone of such a type. In FIG. 12, the reference numeral 1 designates a front case and 2 designates a rear case. They constitute a case for enclosing electronic components like a printed circuit board equipped with ICs or LSIs. The reference numeral 3 designates a liquid crystal display (LCD) mounted on the front case 1; and 4 designates a keypad including dialing keypad and function keys arrayed on the front case 1, which are backlighted by a light source not shown in this figure, for convenience of users operating in dark places. The reference numeral 5 designates a flip rotatably mounted on an edge of the front case 1 to cover and protect the keypad and to prevent erroneous operation while it is closed, although its open state is not shown in this figure. The reference numeral 6 designates a microphone mounted on the flip 5, 7 designates a speaker and 8 designates a transmitting and receiving antenna.
With such an arrangement, the mobile telephone includes in the case a light source for backlighting the keypad 4, which consists of an electroluminescent light (simply called EL from now on). The EL comprises, on a base film, a transparent electrode layer, a luminescent layer, a dielectric layer, a back electrode layer, and an insulating layer, which are stacked in this order, and emits light when supplied with an AC voltage between the transparent electrode layer and back electrode layer to induce electric field excitation for causing electrons to collide with the luminescent layer. Common ELs emit light of a blue-green family.
Thus, in the conventional illuminated keypad, the key buttons are backlighted from their bottom, and are usually provided with numerals, characters or symbols printed on their tops in various colors such as black, red and blue. Accordingly, these prints become clear or opaque, or change their original colors depending on the backlight because the EL emits in the color of the blue-green family, which is unsuitable for some printed colors. This will lead to use white light to backlight the key buttons to improve the visibility of the printed characters.
It will be possible to change the original color of the blue-green family inherent in the EL into white light by mixing red ink, which is a complementary color of the blue green, into the material of the luminescent layer, and by stacking the layer to form the EL as described above. Thus, the backlight becomes white owing to the effect of the red ink.
The white light used to backlight the key buttons will clearly show the numerals, characters and symbols printed on them independently of the printed colors.
The EL thus emitting the white light, however, appears to be pink from the outside through the key button while the EL is inoperative because the red ink is mixed and the key button is composed of light transmission material, thereby spoiling its beauty.
In short, the conventional illuminated key pad is undesirable in terms of the beauty, and provides users with a sense of incongruity at switching the EL between its operative mode and inoperative mode because it appears to be white while it is emitting light in the operative mode, but to be pink while it is in the inoperative mode.